The present invention relates to an air compressor for use in a vehicle, for example. More particularly, the present invention relates to an air compressor suitably used to supply and discharge compressed air for vehicle height adjustment with respect to an air-suspension system or the like that constitutes a vehicle-height adjusting apparatus.
In general, an air-suspension system mounted on a vehicle as a vehicle-height adjusting apparatus is selectively supplied with or exhausted of compressed air from an air compressor to suppress changes in the vehicle height that may occur according to the vehicle weight or the like and to allow vehicle height adjustment to be made as the driver likes.
A vehicle-mounted air compressor used for an air-suspension system or the like has a cylinder and a piston reciprocally provided in the cylinder to compress air in the cylinder. The air compressor further has a cylinder head mounted on the cylinder. The cylinder head is provided with an air supply passage for supplying compressed air generated by the piston to a pneumatic apparatus such as an air-suspension system. In addition, an exhaust valve is provided in the cylinder head to discharge compressed air from the air supply passage to an exterior of the cylinder head [for example, see Japanese Patent Application Unexamined Publication (KOKAI) No. 2-141321 (1990)].
In this type of conventional air compressor, when compressed air is to be supplied to an air-suspension system, for example, the piston is caused to reciprocate in the cylinder with the exhaust valve closed in advance, thereby generating compressed air and supplying it to the air-suspension system through the air supply passage. In the air-suspension system, an air chamber is expanded by the compressed air supplied thereto, and thus vehicle height adjustment is made so as to raise the vehicle height.
During vehicle height adjustment for lowering the vehicle, the exhaust valve is opened, with the piston reciprocating motion in the cylinder stopped, to allow the air supply passage to communicate with the exterior of the cylinder head, thereby making compressed air flow backward from the air chamber of the air-suspension system into the air supply passage. Thus, the compressed air is discharged to the exterior of the cylinder head to contract the air chamber.
In the conventional air compressor, the cylinder head is provided with a single exhaust passage for allowing the air supply passage to communicate with the outside air, and a solenoid-operated exhaust valve that constitutes the exhaust valve is placed at an intermediate position in the exhaust passage. The solenoid-operated exhaust valve is a normally closed valve. Accordingly, the solenoid-operated exhaust valve opens the exhaust passage only when it is opened by externally supplying an electric current thereto, and permits compressed air to be discharged from the air supply passage to the exterior of the cylinder head.
Incidentally, the above-described conventional air compressor is merely arranged such that a single exhaust passage is provided in the cylinder head and the exhaust passage is selectively opened or closed by the solenoid-operated exhaust valve. Therefore, when compressed air in the air-suspension system is discharged to the exterior of the cylinder head to adjust the vehicle height to a lower level, the flow rate of compressed air to be discharged is undesirably limited by the single exhaust passage. Consequently, the compressed air discharge speed is unfavorably low. Therefore, it is difficult to perform vehicle height adjustment in a short period of time.
It is possible to take measures to increase the compressed air discharge speed, for example, by increasing the port diameter of the solenoid-operated exhaust valve. However, if the port diameter of the solenoid-operated exhaust valve is increased, the pressure-receiving area of the valving element with respect to the exhaust port becomes large. Therefore, it becomes necessary to increase the urging force of a valve spring for urging the valving element of the solenoid-operated exhaust valve in a valve closing direction and also necessary to increase the size of a solenoid (coil) for driving the valving element in a valve opening direction against the valve spring. Consequently, not only the solenoid-operated exhaust valve but also the cylinder head must be increased in size.